It is often desirable to add and/or remove components from an electrical system without disrupting the system power. The term "hotswappable" refers to this ability to insert unpowered electronics, or remove powered electronics, from a powered system without disrupting the system power.
Certain problems typically arise when inserting and removing components in a powered electronic system. Inserting an unpowered electronic component may temporarily reduce the system power voltage level below an acceptable limit and cause actively-functioning components in other parts of the system to fail. A voltage reduction occurs because an instantaneous current (the "inrush current") is required to charge the capacitance of the circuits within the newly-inserted component. Uncontrolled inrush current is characterized by a current spike which the system power source is unable to supply, therefore resulting in a voltage drop in the overall system.
FIG. 1 shows a chassis system commonly used in computer and computer networking environments. Main-chassis 1 is a mechanical enclosure which houses one or more power supplies (not shown) and a back-plane circuit card (not shown) which is used to connect and provide power to electronic circuit cards 2-4 and sub-chassis 5. Sub-chassis 5 is a cage which may contain multiple electronic circuit cards (none are shown in FIG. 1), and which can be inserted into the larger main-chassis 1. The electronic circuit cards 2-4 are usually printed circuit cards populated with electronic components. Main-chassis power refers to the power on the main-chassis back plane which is supplied to all of the electronic components residing within the main-chassis.
An exploded view of FIG. 1A shows a single center guide rail 7 mounted on top and bottom portions of the sub-chassis 5, and inner parallel card guides 8 located along top and bottom portions of main-chassis 1. The card guides 8 and center guide rail 7 cooperate to guide the sub-chassis into the main-chassis. Handles 10 are present on the top and bottom of a sub-chassis to assist with insertion and removal. Upon full insertion, the sub-chassis backplane 9 electrically interfaces with the main-chassis backplane.
The main-chassis card guides 8 also allow the insertion of electronic circuit cards 2-4 directly into the main-chassis. In this case, outside edges of the cards slide along the card guides 8, enabling direct electrical interface of the cards and main-chassis backplane upon full insertion.
FIG. 2 shows electronic circuit cards 11-13 being inserted into sub-chassis 5 along sub-chassis card guides 6. An electrical interface is formed between the card and backplane of the sub-chassis when the card is fully inserted. In FIG. 2, sub-chassis 5 has three slots, each containing one of circuit cards 11-13. A "slot" in a sub-chassis or main-chassis is generally defined by the card guides (6 or 8) located on the top and bottom of the chassis. Usually, a single circuit card is said to "occupy" or "be mounted in" a single slot in a chassis when fully inserted. The left-most slot of the sub-chassis in FIG. 2 contains partially inserted circuit card 11. The middle and right-most slots contain fully inserted circuit cards 12 and 13, respectively. A sub-chassis, having one or more circuit cards inserted therein so as to electrically interface with the backplane of the sub-chassis, is said to be "populated". Oftentimes, the slots contained within a chassis (main- or sub-) are numbered from 1-N, where N is the total number of slots which the chassis contains. Those skilled in the art will understand that many different configurations are possible when mounting electronic circuit cards and sub-chassis combinations into a main-chassis.
An unpowered electronic circuit component may be modeled as a single large capacitor for understanding the inrush current phenomena during the application of power. A capacitor has the current/voltage relationship: I=C dv/dt. When a circuit card has power instantaneously applied (emulating an insertion during hotswap), dv/dt (the change in voltage per change in time) is very large; this causes a large instantaneous (inrush) current. The inrush current will disappear as the capacitor is charged. When many circuit cards are placed in a sub-chassis, the draw on the power supply is even greater. Inserting an uncharged sub-chassis containing many circuit cards into a powered main-chassis backplane is similar to a momentary short circuit across the main-chassis power supply due to the instantaneous current required to charge the capacitance of the sub-chassis and its circuit cards. Because the power source cannot respond to this instantaneous current demand, the backplane voltage drops. Once the instantaneous current requirement is satisfied, then the power source will again be able to supply the correct voltage to the backplane; however, because of the prior voltage drop, the functioning of previously-operating circuitry can no longer be guaranteed.
There are various prior art approaches to hotswapping. However, the prior art deals generally with single card hot-swaps. There has not been a satisfactory solution to the problem of hotswapping a populated sub-chassis without disrupting the electronic activity of powered and functioning electronic circuit cards already populated and functioning in a powered main-chassis or sub-chassis.